1. Field of the Invention
This invention relates to ceramic bodies which in use are subjected to stress, and to methods and apparatus for monitoring changes caused by subjecting ceramic bodies to stress in use. The invention can provide a diagnostic method and apparatus for estimating the remaining useful working life of a ceramic body.
2. Description of the Prior Art
One of the biggest problems in application of structural ceramics to actual working parts lies in their low reliability. A ceramic can be superior to a metal in resistance against heat, acid and abrasion, but is less reliable because of low fracture toughness (poor tenacity), creating problems which make it difficult to use the ceramic as structural parts. Many methods of improving toughness (e.g. "Si.sub.3 N.sub.4 --SiC Whisker Based Composite Ceramics," Ceramics No.12, 1983) or a method of confirming the development of cracks ("Non-Destructive Evaluation Technique for Ceramics," Ceramics No.10, 1985) by means of ultra-sonic waves have been proposed to solve these problems.
To determine the maintenance period for a gas turbine plant, for example, the service life is calculated (JP-A-59-3335), or the service life of a test piece placed close to the actual machine part is measured for diagnosis (JP-A-57-113360). All these methods fail to diagnose the state of the actual machine part directly, and cannot be called precise life diagnostic methods.
The prior art discussed above therefore does not diagnose the service life of the actual machine part directly, leaving much room for improvement in reliability prediction and life diagnosis. This is because, despite particle dispersion and fiber reinforcement, the efforts for improving toughness cannot provide fundamental change of the physical properties of the ceramic; apparent toughness may be improved, but its value may be only one fifth of that of a metal. The method of confirming development of cracks by ultra-sonic waves may allow measurement on the test piece level; however, when it is to be applied to actual parts, it is accompanied by problems for use, such as problems of attaching the sensor to the ceramic parts or additional installation of measuring instruments. Reliability cannot be ensured for use of a ceramic by using the method which does not directly diagnose the state of the machine part.